Chapter 14 - Future Development of Lead-Free Piezoelectrics by Domain Wall Engineering

Abstract

Various engineered domain configurations were induced into barium titanate (BaTiO3) single crystals, and their piezoelectric properties were investigated as a function of (1) the crystal structure, (2) the crystallographic orientation, and (3) the domain size (domain wall density). As a result, the orthorhombic mm2 BaTiO3 crystals showed the highest piezoelectric properties among three kinds of BaTiO3 crystals, namely the tetragonal 4mm, orthorhombic mm2, and rhombohedral 3m phases. On the other hand, the [001]c oriented BaTiO3 crystals always exhibited larger piezoelectric properties than the [111]c oriented BaTiO3 crystals. Moreover, we discuss the domain size dependence of the piezoelectric properties, and the result reveals that the piezoelectric property was strongly dependent on the domain size (domain wall density), that is, the piezoelectric properties significantly increased with decreasing domain size (increasing domain wall density). To explain this phenomenon, the multidomain crystals were regarded as the composite of (a) a distorted domain wall region and (b) a normal tetragonal domain region. Using a two-phases model, the piezoelectric properties from the domain wall were estimated. As a result, ultrahigh piezoelectric constants over 80,000pC/N were expected from the 90° domain wall region. Moreover, this study suggested that it is possible to obtain the lead-free piezoelectric materials with the values of d31 and d33 over 1000pC/N, when the domain sizes can be decreased below 1μm. On the basis of the aforementioned results, the most suitable engineered domain configuration was proposed for the high-strain high-coupling piezoelectric applications.